As is known, helicopters are generally equipped with a plurality of transmission assemblies designed to transmit motion from one or more turbines to the rotors, the main rotor and/or the tail rotor, and/or from the turbine to a plurality of accessory devices, i.e., devices responsible, for example, for supplying the energy necessary for operation of the on-board equipment.
In the aeronautics sector, it is likewise known to set a mechanical transmission between the turbine and a drive shaft of the main rotor.
Said mechanical transmissions use generally as last reduction stage an epicyclic motor reducer designed to transmit the power with an adequate torque and r.p.m. to the rotor shaft.
Said epicyclic motor reducer basically comprises:                a first gear, defined as “sun gear”, which can turn about a first fixed axis;        a second, fixed, gear, defined as “crown wheel”, which has a second axis coinciding with the first axis; and        a plurality of gears, defined as “planetary gears”, which mesh with the sun gear and the crown wheel and are able to turn about respective third mobile axes.        
Planetary gear trains further comprise a planetary-gear carrier, which is able to turn about the first axis and is connected to the planetary gears.
In this way, the planetary gears, in addition to turning about the respective third axes, are drawn in a motion of revolution about the first axis of the sun gear by the planetary-gear carrier.
In particular, the sun gear is connected to an input shaft and the planetary-gear carrier is used as output shaft connected to the rotor shaft.
In other words, the mechanical power enters the planetary gear train via the sun gear and is transmitted to the rotor shaft via the planetary-gear carrier, with the right torque and r.p.m.
The pressure angles of the teeth of the planetary gears (and of the sun gear or of the crown wheel) are generally greater than the pressure angles of the teeth of ordinary gear trains. More precisely, the pressure angles of the teeth of the planetary gears (and of the sun gear or of the crown wheel) are generally greater than 25° whilst the pressure angles of the teeth of the gears of ordinary gear trains are 20°.
Planetary gear trains of a known type are moreover conventionally sized in such a way that the number of teeth of the planetary gears is equal to the semidifference between the number of teeth of the crown wheel and of the sun gear. In this way, the pressure angles, i.e., the pressure angles used during generation by envelope, coincide with the angles of working pressure.
With reference to ordinary gear trains, there has been suggested, for example in EP-A-1939493, the use of a pair of gears, which have at least two teeth meshing simultaneously with one another in order to reduce the loads acting on each single tooth of the gear.
However, unlike ordinary gear trains, the teeth of the planetary gears of planetary gear trains must mesh cyclically both with the teeth of the sun gear on a first side and with the teeth of the sun gear on a second side.
Consequently, it is not possible to increase simply the number of teeth meshing between the planetary gears and the sun gear (or crown wheel) without preventing the aforesaid teeth from coming to simultaneously mesh on different sides thereof with the teeth of the sun gear and of the crown wheel.
On account of this, planetary gear trains of a known type have a number of teeth simultaneously meshing between the planetary gears and the sun gear (or between planetary gears and crown wheel) that is substantially equal to one.
Said arrangement is generally accepted in solutions of a known type in so far as the teeth of the planetary gears (and of the crown wheel and sun gear) are subjected to loads lower than the ones to which the teeth of an ordinary gear designed for transmitting the same power are subjected.
In fact, one of the peculiarities of planetary gear trains lies in the fact that the load that passes from the sun gear to the crown wheel is distributed substantially in a uniform way on each planetary gear.
In other words, said load is distributed over the teeth of the planetary gears meshing with respective teeth of the crown wheel and sun gear.
Particularly felt in the aeronautics sector is the need to obtain a planetary gear train that will increase as much as possible the ratio between the power transmitted and the weight of the gear train itself.
Said ratio is evidently of greatest importance in the aeronautics sector.
Finally, also felt in the sector is the need to render the transmission of motion to the rotor drive shaft as quiet as possible.